The present invention relates to a comparator or its forming method, and more particularly to an offset comparator and its forming method.
As well known in the art, the resistor is generally used to create an offset or a threshold or trip voltage in a comparator. FIG. 1 shows a comparator with a threshold offset caused by using a resistor. The offset comparator includes a differential stage 30 having a first input V01 and a second input V02, an output stage 40 electrically connected to differential stage 30 such that the output V0 is zero when two inputs V01 and V02 have therebetween a specific voltage difference. A biasing stage 50 having two source followers SF1 and SF2 being equally biased by Vb1 to be respectively electrically connected to inputs V02 and V01 of differential stage 30. A resistor 14 is electrically connected in source follower SF2 for providing an offset in the comparator. Transistors 12 and 13 constitute the first source follower SF1 where the voltage at input V02 is a threshold below the comparator input Vin2. Transistors 11 and 10 and resistor 14 constitute a second source follower SF2 in which the voltage at the point Vr1 is a threshold below the comparator input Vin1 and the voltage at differential input V01 is an IR drop below the voltage at point Vr1. The offset in the comparator transfer function is created due to resistor 14 which creates an offset between differential inputs V01 and V02.
For a threshold offset comparator, output V0 is non-zero when both inputs V01 and V02 are equal. Hence, an offset needs to be created between two inputs V01 and V02 of differential stage 30 when the two inputs Vin1 and Vin2 to source followers SF1 and SF2 are equal, in order to create an offset in the transfer function of differential stage 30.
As mentioned above, the resistor is commonly used to generate an offset or a trip voltage in the comparator. When both comparator inputs Vin1 and Vin2 are equal, the outputs V01 and V02 of source followers SF2 and SF1 are deliberately made unequal. When one of comparator inputs Vin1 and Vin2 exceeds the other input by a certain value, source follower outputs or differential stage inputs V01 and V02 are equal. That value is called the trip voltage of the comparator and the output V0 of the comparator is zero in this value. The above-mentioned mode of operation creates an offset in the comparator output.
Resistor 14 is the key to creating an offset in the comparator transfer function. The variation of the resistor across different process corners and temperatures, however, will cause the comparator threshold voltage to vary. In the past, compensated resistors are proposed to overcome this situation. The resistor compensation, however, will require some complex circuit design. This will lead to an increased silicon area and more power consumption. Moreover, resistors have parasitic capacitances associated therewith which will lead to a reduced bandwidth.
It is therefore tried by the Applicant to deal with the above situations encountered in the prior art.
It is therefore an object of the present invention to provide a method of forming a comparator for alleviating problems associated with a comparator using a resistor.
It is further an object of the present invention to provide a resistorless threshold offset comparator.
In addition, an object of the present invention to provide an offset comparator having a resistorless bias compensation.
It is still an object of the present invention to provide an offset comparator having a threshold voltage independent of different process corners and/or temperatures.
It is yet an object of the present invention to provide an offset comparator having a reduced silicon area.
It is one more object of the present invention to provide an offset comparator being better in terms of portability and/or area efficiency.
It is again an object of the present invention to provide an offset comparator suitable for use in an increased bandwidth.
It is once more an object of the present invention to provide an offset comparator in which the process compensation is automatically implemented as a part of the bias circuit.
It is nevertheless an object of the present invention to provide an offset comparator having a simple compensation technique.
It is furthermore an object of the present invention to provide a compensated offset comparator having a less power consumption.
According to an aspect of the present invention, an offset comparator includes a differential stage having a first input and a second input, an output stage electrically connected to the differential stage such that the output is zero when the two inputs have therebetween a specific voltage difference, and a biasing stage electrically connected to the differential stage, providing a first biasing voltage for creating a second input voltage in the second input and providing a second biasing voltage for creating a first input voltage in the first input such that the two input voltages have therebetween the specific voltage difference.
The biasing stage generally includes a first voltage provider for providing the second input voltage, and a second voltage provider for providing the first input voltage.
Certainly, the first and second voltage providers can respectively be a first source follower and a second source follower. The second biasing voltage can be lower than the first biasing voltage, or alternatively, the first biasing voltage lower than the second voltage.
Preferably, the comparator further includes a first diode connected transistor electrically connected to the first source follower for providing thereto a first biasing current, and a second diode connected transistor electrically connected to the second source follower for providing thereto a second biasing current.
Certainly, the first biasing current can be lower than the second biasing current, or alternatively higher than the second biasing current.
The present comparator can further include a diode connected transistor electrically connected to the second voltage provider for providing thereto a relatively constant biasing current. Alternatively, the present comparator preferably includes a bias-compensating circuit electrically connected to the second voltage provider so that the comparator can produce a fixed trip voltage.
Certainly, the bias-compensating circuit can include a fixed biasing current circuit for providing a fixed biasing current, and a variable current circuit electrically connected to the fixed biasing current circuit for providing a manufacturing parameter-dependent current.
Preferably, the fixed biasing current is a diode connected transistor obtaining the fixed biasing current from a bias circuit. The variable current circuit includes a complementary pair of transistors provided with a fixed bandgap voltage.
Preferably, the present comparator further includes a current mirror electrically connected to the variable current circuit for providing the second voltage provider with a current being a summation of the fixed biasing current and the variable current.
Preferably, the present comparator further includes a diode connected transistor electrically connected between the current mirror and the second voltage provider.
In accordance with a second aspect of the present invention, an offset comparator includes a differential stage having a first input and a second input, an output stage electrically connected to the differential stage such that the output is zero when the two inputs have therebetween a specific voltage difference, and a resistorless biasing stage electrically connected to the differential stage, providing a first input voltage for the first input and providing a second input voltage for the second input such that the two input voltages have therebetween the specific voltage difference.
Certainly, the biasing stage can include a first voltage provider for providing the second input voltage, and a second voltage provider for providing the first input voltage. The first and second voltage providers can respectively be a first source follower and a second source follower.
Preferably, such comparator further includes a bias-compensating circuit electrically connected to the second voltage provider so that the comparator can produce a fixed trip voltage.
Certainly, the bias-compensating circuit can include a fixed biasing current circuit for providing a fixed biasing current, and a variable current circuit electrically connected to the fixed biasing current circuit for providing a manufacturing parameter-dependent current.
Preferably, such comparator further includes a current adder electrically connected to the variable current circuit for providing the second voltage provider with a current being a summation of the fixed biasing current and the variable current.
Certainly, the current adder can be a current mirror. The manufacturing parameter-dependent current depends on a specific manufacturing process and a specific manufacturing temperature.
In accordance with a third aspect of the present invention, an offset comparator includes a differential stage having a first input and a second input, an output stage electrically connected to the differential stage such that the output is zero when the two inputs have therebetween a specific voltage difference, a first voltage provider electrically connected to the second input for providing therewith a second input voltage, a second voltage provider electrically connected to the first input for providing therewith a first input voltage, and a biasing circuit electrically connected to the first and second voltage providers for respectively differentially biasing therefor such that the two input voltages have therebetween the specific voltage difference.
Preferably, the biasing circuit provides a first biasing voltage to the second voltage provider and a second biasing voltage to the first voltage provider.
Preferably, this comparator further includes a bias-compensating circuit electrically connected to the second voltage provider so that the comparator can produce a fixed trip voltage. The bias-compensating circuit includes a fixed biasing current circuit for providing a fixed biasing current, and a variable current circuit electrically connected to the fixed biasing current circuit for providing a manufacturing parameter-dependent current.
Preferably, the fixed biasing current is a diode connected transistor obtaining the fixed biasing current from a bias circuit. The variable current circuit includes a complementary pair of transistors provided with a fixed bandgap voltage.
Preferably, this comparator further includes a current adder electrically connected to the variable current circuit for providing the second voltage provider with a current being a summation of the fixed biasing current and the variable current.
In accordance with a fourth aspect of the present invention, a method for forming an offset comparator comprising the steps of a) providing a differential stage having a first input and a second input, b) providing an output stage electrically connected to the differential stage such that the output is zero when the two inputs have therebetween a specific voltage difference, and c) providing a biasing stage which has a characteristic dependent on a manufacturing parameter, is electrically connected to the differential stage, provides a first biasing voltage for creating a second input voltage in the second input and provides a second biasing voltage for creating a first input voltage in the first input such that the two input voltages have therebetween the specific voltage difference independent of the manufacturing parameter.
Certainly, the manufacturing parameter can include a manufacturing process corner and a manufacturing temperature. The biasing stage includes a first voltage provider for providing the second input voltage and a second voltage provider for providing the first input voltage.
Preferably, the present method further includes a step of d) providing a bias-compensating circuit electrically connected to the second voltage provider for providing for the comparator the specific voltage difference. The bias-compensating circuit includes a fixed biasing current circuit for providing a fixed biasing current, and a variable current circuit electrically connected to the fixed biasing current circuit for providing a manufacturing parameter-dependent current.
Preferably, the present method further includes a step of e) providing a current adder electrically connected to the variable current circuit for providing the second voltage provider with a current being a summation of the fixed biasing current and the variable current.